The present invention relates to a plane emission type semiconductor laser device and a method of manufacturing the same, particularly to a plane emission type semiconductor laser device including a current confinement layer which is so constituted as to have excellent in-plane uniformity and a method of manufacturing the same.
A plane emission type semiconductor laser device is a semiconductor laser device which includes a pair of reflectors composed of a multiplicity of compound semiconductor layers and an active layer (light emission layer) provided between the reflectors, on a substrate, and which emits a laser beam in a direction orthogonal to the substrate. Since the plane emission type semiconductor laser device is greatly advantageous to mounting, it has been vigorously researched and developed.
In order to enhance the injected current-light output efficiency, the plane emission type semiconductor laser device is provided with a current confinement structure in which the flow path for a current injected from an electrode is confined so as to forcibly inject the current concentratedly into a light emission region of the active layer. Among others, an oxidized confinement type current confinement layer composed of an Al oxide layer formed by oxidizing a high-Al-content layer has been paid attention to, since it is easy to form. The term “oxidized confinement type” means that the oxidized portion is thereby made insulating, i.e, made to block the flow of currents, so that a confinement structure can be formed.
For example, Japanese Patent Laid-open No. 2001-284727 proposes a configuration of a plane emission type semiconductor laser device capable of efficiently emitting a laser beam with a substantially true circular beam profile in a single transverse mode (FIG. 2).
Here, referring to Japanese Patent Laid-open No. 2001-284727, and by using FIG. 3, the configuration of a plane emission type semiconductor laser device having a general oxidized confinement type current confinement structure according to the related art will be described. FIG. 3 is a sectional view showing the configuration of the plane emission type semiconductor device having the general oxidized confinement type current confinement structure according to the related art.
As shown in FIG. 3, the conventional plane emission type semiconductor laser device 10 includes, on an n-type semiconductor substrate 12, a laminate structure of a lower reflector 14 composed of a multiplicity of n-type compound semiconductor layers, a lower clad layer 16, an active layer 18, an upper clad layer 20, an upper reflector composed of a multiplicity of p-type compound semiconductor layers, and a p-type contact layer 24.
Of the laminate structure, the contact layer 24, the upper reflector 22, the upper clad layer 20, the active layer 18, the lower clad layer 16, and an extreme upper portion of the lower reflector 14 are formed as a mesa post 26.
In addition, a p-side electrode 28 is provided on the p-type contact layer 24, while an n-side electrode 30 is provided on the back side of the n-type semiconductor substrate 12, and an insulation film 32 is formed on the side surface of the mesa post 26 and on the lower reflector 14 in the vicinity of the mesa post 26.
In a layer, near the active layer 18, of the upper reflector 22, there is formed a current confinement layer 34 in place of the compound semiconductor layer constituting the upper reflector 22. The current confinement layer 34 is constituted of a high-Al-content layer 34A present in a central region, and an Al oxide layer 34B formed in an annular shape along the circumference of the mesa post 26 so as to surround the high-Al-content layer 34A.
The Al oxide layer 34B is an AlOx layer formed by selective oxidation of a high-Al-content layer such as an AlAs layer, a high-Al-content AlGaAs layer (Al content>0.95), etc., and constitutes a current confinement region which has a high electric resistance. The high-Al-content layer 34A is an unoxidized AlAs layer or high-Al-content AlGaAs layer (Al content>0.95), and constitutes a current injection region which is lower than the Al oxide layer 34B in electric resistance.
With the above-mentioned configuration, an electric current injected from the p-side electrode 28 toward the n-side electrode 30 is confined by the current confinement region 34B, passes through the current injection region 34A, and flows locally concentratedly into a light emission region of the active layer 18, to generate an inverted distribution of carriers in the active layer 18, whereby laser oscillation can be effected.
At the time of manufacturing the plane emission type semiconductor laser device 10, first, the laminate structure of the lower reflector 14, the lower clad layer 16, the active layer 18, the upper clad layer 20, the upper reflector 22 including the high-Al-content layer, and the p-type contact layer 24 is formed on the n-type semiconductor substrate 12, and then the laminate structure is etched to form the mesa post 26.
Subsequently, the laminate structure provided with the mesa post 26 is fed into an electric furnace filled with a water vapor atmosphere, and is heated to about 400° C. to oxidize the high-Al-content layer inwards from the circumference of the mesa post 26, whereby the annular Al oxide layer 34B is formed while leaving the unoxidized high-Al-content layer 34A in the central region as the current injection region.
Meanwhile, a high-Al-content layer such as an AlAs layer intrinsically has a high electric resistance, even though it is not oxidized. Therefore, in determining the device resistance of the plane emission type semiconductor laser device, it is necessary to control the area of the unoxidized high-Al-content layer (current injection region) to a predetermined area.
Accordingly, in order to control the area of the current injection region to the predetermined area, it is necessary to control the oxidation width of the Al oxide layer, i.e., the oxidation width of the annular oxide film formed along the circumference of the mesa post. In addition, in order to control the transverse mode of the light emitted from the plane emission type semiconductor laser device, also, it is necessary to control the oxidation width of the Al oxide layer.
At the time of oxidizing the high-Al-content layer such as an AlAs layer, as above-mentioned, the laminate structure of the compound semiconductor layers is etched to form an air post type mesa post, and the water vapor oxidation reaction of the high-Al-content layer is made to proceed inwards from the side surface of the mesa post.
According to the conventional method, however, the control of the oxidation width is carried out by controlling the reaction time, and, therefore, the repetitive reproducibility of the oxidation width and in-plane uniformity have not necessarily been satisfactory; generally, the repetitive reproducibility of the oxidation width has been about ±1 μm, and the in-plane distribution has also been about ±1 μm.
For control of the device resistance and the transverse mode, generally, it is necessary to control the oxidation width to a predetermined value in the range of from several micrometers to ten and several micrometers. However, since the repetitive reproducibility of the oxidation width has been about ±1 μm and the in-plane distribution has been about ±1 μm, it has been difficult to form a current confinement region with an accurate oxidation width, i.e., a current confinement layer having a current injection region with a predetermined area.
According to the related art, therefore, the laser characteristics of the plane emission type semiconductor laser device would be scattered, and it is difficult to enhance the yield of the product.